356 RESPIRATION 



It is usually not till the oxygen pressure in the air reaches more 

 than three atmospheres that warm-blooded animals show marked 

 immediate symptoms of oxygen poisoning. This we can under- 

 stand. The extra oxygen taken up in the arterial blood is nearly 

 all in simple physical solution, as Paul Bert showed by blood-gas 

 analyses of the arterial blood. At three atmospheres of oxygen the 

 blood will only take up about seven volumes of oxygen in solution. 

 On the other hand, the blood commonly loses about as much oxy- 

 gen in its passage through the capillaries. It is also indicated 

 by the results of experiments described in Chapter X, that the 

 effect of the increased oxygen is to slow the circulation, so 

 that more oxygen than usual is lost. Hence the oxygen pres- 

 sure will probably be very little above normal in the tissues 

 or venous blood until the oxygen pressure in the arterial blood 

 is over three atmospheres. As was shown in Chapter VII, ani- 

 mals in which the haemoglobin has been thrown out of action 

 by CO or nitrite poisoning are still a little short of oxygen when 

 they are breathing oxygen at two atmospheres pressures. We can 

 therefore easily understand why so high an oxygen pressure as 

 three or four atmospheres is needed before the nervous system 

 and other tissues are markedly affected by the oxygen. 



In his experiments on warm-blooded animals Paul Bert had, 

 however, overlooked one thing which his other experiments might 

 have led him to look for. Although the tissues generally in a 

 higher animal are protected from the high pressure of oxygen, 

 since they have round them that wonderfully constant internal 

 environment which protects them from so many variations in the 

 external environment, yet the cells lining the air passages and 

 lungs are exposed to the high oxygen. It was discovered by Lor- 

 rain Smith in i899 10 that oxygen at a pressure quite insufficient 

 to affect the nervous system appreciably will, if time is given, 

 produce fatal inflammation of the lungs. The higher the pressure 

 of the oxygen, the sooner this appears. The lungs are filled with 

 exudation, so that they sink in the fixing fluid, a general oedema 

 similar to that in phosgene poisoning being produced. Probably 

 the animals only survive as long as they do in the compressed 

 oxygen because they get sufficient oxygen in spite of the oedema. 

 As Lorrain Smith showed, the oedema protects them against the 

 effects of very high oxygen pressure on the nervous system. At an 

 oxygen pressure of 180 per cent of an atmosphere (that to which 



10 Lorrain Smith, Journ. of Physwl., XXIV, p. 19, 1899. 



